The elimination of used tires is becoming an increasingly difficult problem throughout the world. As an example of the importance of the problem, over 200,000 tons of waste tires and tubes are produced each year in Canada. In the United States of America, that amount is close to 4,000,000 tons.
Although it is very attractive to consider the possibility of using waste rubber products such as tires or tubes in asphalt (or bitumen) compositions, there are a number of significant obstacles related to such use. Scrap tires contain different types of vulcanized rubber, for example polyisoprene, polybutadiene, styrene-butadiene, butyl, ethylene-propylene-diene, as well as fillers, plasticizers, surfactants and the like. Sometimes, tires contain up to 31 different elastomers. Ground rubber or crumb rubber does not disintegrate readily, because of the crosslinking of the polymeric material that takes place by the vulcanization process with sulfur. The additional chemical bonds created during this vulcanization render impossible the complete dissolution of the polymer in hot bitumen, thus causing problems of stability and storage of the material. Furthermore, such mixtures are not homogeneous, and form two phases of different density which are partially dispersed one in the other. The system tends to decant and causes the upper layers, which have a lower density, to be constituted of polymer, while the lower layers, which have a higher density, are constituted of bituminous material.
At high temperature, conventional asphalt compositions, that is asphalt cement for use in pavement which meet ASTM-D-946-74 or ASTM-D-3381-83 specifications, become fluid and even sticky, causing aggregate deformation or rutting, while at lower temperatures, they become fragile and crack easily. Toughness and durability of asphalt compositions are directly proportional to the resistance of the asphalt to deformation under the effect of stress.
DE 38 19 931 discloses polymer-modified bitumen compositions comprising bitumen in admixture with an olefin, the latter having been oxidized before being mixed with bitumen. These compositions are alleged to be binders in hot-mix asphalt such as pavements. These compositions, as useful as they could be, are however extremely expensive, because of the use of virgin olefins, that is, brand new synthetic olefins which have never been used before.
CA 1,109,191; CA 1,209,295; CA 1,137,242 and CA 1,137,243 (corresponding to U.S. Pat. Nos. 4,145,322; 4,554,313; 4,567,222; 4,242,246 and 4,330,449) disclose processes of preparation of asphalt compositions which comprise mixing vulcanized virgin olefins with conventional bitumen. The vulcanization is performed with sulfur or polymeric sulfur derivatives. As explained above, such compositions are extremely costly because of the use of virgin olefins.
U.S. Pat. No. 4,992,492 discloses a bitumen and reclaimed rubber powder binder for surfacing roadways. The material and the process for preparing it are however limited by the size of the particles of reclaimed rubber. If the size of the particles is too important, the resulting mixture, or binder, is unstable, non homogeneous, and cannot be stored. It is also obvious that the smaller the particles of reclaimed rubber are, the higher their cost.
U.S. Pat. No. 4,437,896 is concerned with synthetic bitumen compositions comprising gilsonite, tall oil and oil pitch. Again the size of the reclaimed rubber particles that can be used in the process of this patent cannot exceed 850 microns, which constitutes an important drawback.
U.S. Pat. No. 4,609,696 teaches asphalt emulsion, which may contain reclaimed rubber. The particles involved in the process are also limited to a size of 850 microns, and preferably 300 microns.
It would therefore be highly desirable to obtain an asphalt composition having good impact resistance at low temperature and excellent rheological and mechanical properties. Improving these properties over a range of temperatures provides a superior load bearing capacity of pavement, and a better ability to resist to ground movements or other stresses. Waste rubber such as waste tires and tubes would preferably be used in the composition. Preferably, such asphalt composition would be obtained through a versatile and inexpensive process. In order to further reduce the costs of such asphalt composition, it would be necessary that it be prepared with rubber particles of the greatest possible size.